Lyapunov Exponent

A real number quantifying the average exponential rate at which nearby trajectories of a dynamical system separate (positive) or converge (negative) along a given direction.

A quantitative rate that characterizes the average exponential divergence or convergence of infinitesimally close trajectories in a dynamical system, typically defined as the time-asymptotic limit of the growth rate of small perturbations measured by the logarithm of a tangent map or variational flow.